In general, as pixels made using CMOS processes for image sensors scale to smaller dimensions, several performance properties of the imagers using these pixels degrade. One performance property in particular, quantum efficiency (QE), degrades quickly. The loss in performance is confounded with the addition of a color-filter-array (CFA) on top of the pixel array. The purpose of the CFA is to allow for color separation of the incoming light for providing the ability to reconstruct color images. However, for a given wavelength, most of the filters are absorbing. Therefore, any given wavelength effectively sees a series of small apertures above the pixel array. As the pixel pitch shrinks, the size of this effective aperture in the CFA pattern becomes comparable to the wavelength of visible light. Light diffraction diverts light onto adjacent pixels and reduces the effective QE of the targeted color pixel. Consider FIG. 1a for example. For incoming red light, the blue and green CFA of the blue 103 and green pixels 101, 104 are effectively blocking. For a Bayer pattern 105, FIG. 1b illustrates this creates a small aperture 112 above the red pixel 102 for red light. Especially below 2 μm pixel pitches, diffraction spreads the incoming red light into the adjacent blue and green pixels since the CFA is positioned a finite distance above the active layer of the image sensor where the photons are converted to charge carriers. Diffraction corrupts the effectiveness of the CFA to separate colors, increasing color crosstalk. It also effectively reduces the QE of the red pixel.
FIG. 2 shows prior art for the cross-section of four pmos pixels through the red and green CFA of a back illuminated image sensor. This will also be used as a reference point for describing the present invention in the Detailed Description of the Invention.
Still referring to FIG. 2, there is shown a photodiode 200 where photo-generated charge carriers are collected. For readout the charge carriers are electrically transferred to a floating diffusion 205 by adjusting the voltage on a transfer gate 201. The floating diffusion signal feeds the input of the source-follow transistor 203. The low-impedance output of the source-follower 203 drives the output line 204. After readout the signal in the floating diffusion 205 is emptied into the reset drain 213 by controlling the voltage on the reset gate 202. Sidewall isolation 210 between the photodiodes directs photo-generated charge carriers into the nearest photodiode 200 reducing color crosstalk within the device layer. To reduce dark current there is a thin pinning layer 212 at the surface between the silicon and dielectric near the photodiode 200. To also reduce dark current, there is a thin n-doping layer 211 along the sidewall isolation 210. Incoming light 250 first passes through the color filter array layer 230, then an antireflection coating layer 222, then a spacer layer that is typically silicon dioxide 221 before reaching the active device layer 220. However, the optical stack 221, 222, and 230 can consist of more or fewer layers depending on application, and often includes a micro-lens array for the top layer. FIG. 3 provides a single pixel schematic for this non-shared pinned photodiode structure of FIG. 2.
FIG. 4 shows simulation results for QE for a prior art 1.1 μm pixel array with a Bayer pattern. The peak QE for the blue response curve 503 associated with the blue pixel 103 is 40%. The peak QE for the green response curves 501, 504 associated with the green pixels 101, 104 is 35%. The peak QE for the red response curve 502 associated with the red pixel 102 is 23%. For these simulations the thickness of the dielectric spacer 221 layer is 0.5 μm. Increasing the dielectric spacer thickness 221 degrades performance resulting in lower peak QE and increased color crosstalk.
Although the presently known and utilized image sensor is satisfactory, there is a need to address the above-described drawback.